1. Field of the Invention
The present invention relates to a planar lightwave circuit module for an optical fiber amplifying device suitable for use in optical fiber communications, and to the aforementioned optical fiber amplifying device.
2. Description of Related Art
An optical fiber amplifier is described in "Optical Fiber Amplifiers and Their Use" (Chapter 5: Erbium-doped Fiber Amplifier (EDFA); by Hideki Ishio, Ohm Publishing House, pp. 110-111) (Reference 1).
The structure of this conventional optical fiber amplifier (or amplifying device) is shown in FIG. 1. A brief description of the conventional optical fiber amplifying device will be given with reference to FIG. 1.
In FIG. 1, part of a signal light input is separated by an optical coupler 131 and provided to a photodetector (hereinafter abbreviated as "PD") 132. The remaining signal light is provided to a wavelength division multiplexer (hereinafter abbreviated as "WDM") 134.
In WDM 134, multiplexing is performed between signal light provided by the optical coupler 131 and forward pumping light provided by a laser diode (hereinafter abbreviated as "LD") 135 serving as an a pumping source, and the resultant multiplexed signal is provided to an amplifying optical fiber 136 through an optical isolator 133. In WDM 137, backward pumping light provided by LD 138 is multiplexed with signal light and the resultant multiplexed signal is provided to the amplifying optical fiber 136.
The amplifying optical fiber 136 is provided with the signal light and forward pumping light from WDM 134 through the optical isolator 133 and with the backward pumping light from WDM 137, and the signal light is amplified. Pumping light is removed from the amplified signal light by WDM 137, and the resultant amplified signal light is provided to an optical coupler 140 through an optical isolator 139.
Most of the signal light provided to the optical coupler 140 is outputted to an output terminal, and the remaining signal light is provided to PD 142. Part of the reflected light input from the output terminal is separated by the optical coupler 140 and provided to PD 141.
As noted above, various optical components (for example, optical couplers, WDM, optical isolators, PD, and LD) are used for conventional optical fiber amplifying devices, and optical fibers (hereinafter referred to as "pigtails") are generally used to transmit light to and from the input/output units of such optical components.
With conventional optical fiber amplifying devices, however, pigtails are used for the input/output units of the optical components constituting these devices, and connections among the components are formed using these pigtails, thereby creating a need to accommodate such optical components, connections among them, and pigtail fibers. This arrangement is disadvantageous in that it yields a bulky device. Another drawback is the need to perform work involved in the mounting of the optical components, connections among them, and pigtail fibers, thereby increasing the number of manufacturing steps.
An alternative to using such pigtails is to directly connect the exit surface of an optical component and the incident surface of another optical component, and to use a composite optical component obtained by the partial or complete integration of these components. Using such composite optical components is preferred as a way of overcoming the aforementioned shortcomings.
Because of the absence of light-transmitting pigtails in such composite optical components, however, a plurality of optical components must be fixed at appropriate positions in relation to a single optical collimator (parallel beam) system. Specifically, the optical axes of optical components must be aligned with each other. As a result, fine adjustment or the use of high-precision components is required, making this approach disadvantageous because of an increase in the number of manufacturing steps and a higher cost.
Furthermore, constructing erbium-doped fibers (light-amplifying fibers) in the form of optical waveguides was studied by T. Kitagawa ("Rare-earth Doped Planar Waveguide Amplifiers," Proc. of Topical Meeting Optical Amplifiers and Their Applications, MC1, pp. 136-139 (1993) (Reference 2)). This construction is suitable for miniaturization. This construction is still disadvantageous, however, in that a phenomenon called concentration extinction occurs because of increased erbium density, and optical amplification characteristics are adversely affected by this phenomenon.
Another feature of optical fiber amplifying devices is that a large number of optical components other than light amplification fibers are mounted in a narrow space. Configuring these optical components as planar lightwave circuit modules would make them interchangeable independent of the type of optical fiber amplifying device.